1. Field of the Invention
The present invention relates to a circulation treatment system and method for a fountain solution used in offset printing.
2. Description of the Related Art
In offset printing, control of a fountain solution is essential. A fountain solution circulation system constantly supplies a clear fountain solution containing each ingredient at a respective predetermined concentration to a vessel in a printing unit at a regular flow rate and temperature, collects the fountain solution which has passed through the vessel to restore the fountain solution to be clear, and supplies the fountain solution to the vessel again. Most of the recent offset rotary presses employ continuous circulation systems for fountain solutions. In order to stabilize water intake, isopropyl alcohol (IPA) is used as an organic solvent.
The addition of IPA to a fountain solution decreases surface tension of the fountain solution and enables nonprinting parts to be uniformly moistened with a small amount of fountain solution. Namely, IPA improves wettability of a plate surface, thereby maintaining printing quality. On the other hand, the use of IPA may cause some safety and health problems for workers. Therefore, the amount of IPA used is limited to as small an amount as possible, or a fountain solution which does not include any IPA is used. In any case, control of a fountain solution is becoming more important.
FIG. 5 is a flow diagram showing a fountain solution circulation system 101 of a prior art.
In FIG. 5, a raw fountain solution, water, an alcohol, etc. are mixed at precise proportions in a fountain solution mixer 102, and the mixed fresh fountain solution is supplied to the fountain solution circulation system 101. The fountain solution circulation system 101 removes contaminant in the fountain solution which has returned from a vessel 103 in a printing unit with a filtering device 104, and feeds solution back to a tank (not shown) inside the fountain solution circulation system 101.
When the level of the fountain solution in the tank lowers, a fresh fountain solution is added from the fountain solution mixer 102 through a float valve. The fountain solution in the tank is kept at a predetermined temperature by a cooling device (not shown). An alcohol concentration controller 106 controls the concentration of alcohol, which easily evaporates. The fountain solution in the tank is continuously fed into the alcohol concentration controller 106. The concentration of a hydrogen ion is detected by constantly monitoring the pH of the fountain solution, and the fountain solution is supplemented with a required amount of alcohol to keep the predetermined alcohol concentration.
When an etching solution is used as an alternative of IPA, it is also necessary to maintain the concentration and evaporation rate of the fountain solution by controlling the temperature and concentration. In the case of alcohol, the concentration is controlled using a hydrometer. On the other hand, in the case of an etching solution used as an alternative of IPA, the concentration can be controlled by use of the fact that the conductivity of a fountain solution is proportioned to the concentrations of an etching solution and a concentrated alkali solution. Sometimes organic substances generate in the fountain solution circulation system 101 and change the quality of the fountain solution. In addition, a fountain solution contains contaminant such as ink, paper particles and oil. Thus, it is important to keep the fountain solution circulation system 101 clean as a whole.
Nevertheless, in the above-described fountain solution circulation system 101 of the prior art, a porous filter material such as a sponge filter is used in the filtering device 104, which removes the contaminant of the fountain solution returned from the vessel 103 through a porous layer of the filter only by mechanical filtering. Accordingly, the ability of removing impurities in the fountain solution depends on the dimension of the pores of the filter and, normally, impurities having a diameter less than 100 .mu.m cannot be removed.
If one tries to improve the ability of removing impurities by employing pores having a smaller dimension, a pressure posed on the fountain solution passing through a porous layer prevents the fountain solution from circulating at a predetermined flow rate. If the fountain solution does not circulate at a predetermined flow rate, the fountain solution loses normal functions, leading to deterioration of the printing quality. Furthermore, too small pores of the filter are easily clogged with impurities, which requires frequent maintenance of the filter.
The prior art fountain solution circulation system 101 has further disadvantages due to the limitation of the filtering ability. Specifically, after circulating a fountain solution in the system for a few months, the fountain solution is contaminated and decomposed, and the quality of the fountain solution finally changes to the extent that it cannot be used as a fountain solution. Then, the fountain solution needs to be replaced. In many cases, a discarded fountain solution which has only been filtered by conventional mechanical filtering cannot meet legal standards for drainage of wastewater in terms of a biochemical oxygen demand (BOD), chemical oxygen demand (COD), n-hexane concentration and suspended solid (SS). Thus, the discarded fountain solution is regarded as an industrial waste, and wastewater of a fountain solution cannot be directly drained into rivers or the like. As a result, such waste disposal is committed to specialist companies at considerable cost.
According to the present invention, provided are a circulation treatment system and method for a fountain solution wherein the amounts of a BOD, COD, n-hexane concentration and SS in a fountain solution are reduced without posing any pressure on the fountain solution, thereby recycling the fountain solution without draining off.